This summer was the fourth in a row with ice extents sharply below the long-term average, said Mark Serreze, a senior scientist at the [National Snow and Ice Data Center] and a professor at the University of Colorado in Boulder.

A natural cycle in the polar atmosphere, the Arctic Oscillation, that contributed to the reduction in Arctic ice in the past was not a significant factor right now, he said, adding that rising temperatures driven by accumulating greenhouse-gas emissions had to be playing a role.

“What we’re seeing is a process in which we start to lose ice cover during the summer,” he said, “so areas which formerly had ice are now open water, which is dark.

“These dark areas absorb a lot of the Sun’s energy, much more than the ice; and what happens then is that the oceans start to warm up, and it becomes very difficult for ice to form during the following autumn and winter.

“It looks like this is exactly what we’re seeing ? a positive feedback effect, a ‘tipping-point’.”

The idea behind tipping-points is that at some stage the rate of global warming would accelerate, as rising temperatures break down natural restraints or trigger environmental changes which release further amounts of greenhouse gases.

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